In this supplementary research request we propose to expand the scope and accelerate the rate of progress on our project GM083926, entitled "Regulation of Cellular Functions by Cyclic Nucleotide Phosphodiesterase 8". The parent grant contains 3 specific aims, to determine the functions and mechanisms of action of PDE8s in adrenal, liver, and brown fat tissues. These are three of the tissue types in which this PDE isozyme is highly expressed. Recently, we have found that PDE8A is also highly expressed in cardiac myocytes. Moreover, our preliminary data strongly suggest that ablation of PDE8 can modulate calcium handling in isolated cardiocytes. The focus of the proposed studies will be to determine the overall cardiac phenotype of the PDE8A knockout mouse and to identify the mechanism(s) by which PDE8 regulates calcium transients in the heart. Emphasis will be placed on the roles of PDE8A in controlling cAMP dependent phosphorylation events in isolated cardiomyocytes. We will also study the effects of the gene disruption on calcium regulation in isolated cardiomyocytes and on in vivo measurements of cardiac function. The mission of the Institute of General Medicine is clearly relevant to studies on the mechanisms of control of cardiac function by modulation of calcium in the heart. The possibilities for developing an anti-arrhythmic drug based on modulation of PDE8 activity would seem quite real. PUBLIC HEALTH RELEVANCE: In this supplementary research request we propose to expand the scope and accelerate the rate of progress on our project GM083926, entitled "Regulation of Cellular Functions by Cyclic Nucleotide Phosphodiesterase 8". The focus of the proposed studies will be to determine the overall cardiac phenotype of the PDE8A knockout mouse and to identify the mechanism(s) by which PDE8 regulates calcium transients in the heart. Emphasis will be placed on the roles of PDE8A in controlling cAMP-dependent phosphorylation events in isolated cardiomyocytes. We will also study the effects of the gene disruption on in vivo measurements of cardiac function. These studies are potentially directly relevant to drug regulation of cardiac function. For example, if validated by these studies, it would appear that a drug that acted as a PDE8 activator would likely have anti-arrhythmic activity. Since most heart attacks have an arrhythmia component the relevance to public health is obvious.